A significant number of complex repair jobs carried out by Ace Repairs have to be reworked under the company's warranty. The reworked jobs are invariably satisfactory. When initial repairs are inadequate, therefore, it is not because the mechanics lack competence; rather, there is clearly a level of focused concentration that complex repairs require that is elicited more reliably by rework jobs than by first-time jobs.

The argument above assumes which of the following?

• AThere is no systematic difference in membership between the group of mechanics who do first-time jobs and the group of those who do rework jobs.  
• BThere is no company that successfully competes with Ace Repairs for complex repair jobs.  
• CAce Repairs' warranty is good on first-time jobs but does not cover rework jobs.  
• DAce Repairs does not in any way penalize mechanics who have worked on complex repair jobs that later had to be reworked.  
• EThere is no category of repair jobs in which Ace Repairs invariably carries out first-time jobs satisfactorily.